In recent years, in conjunction with the spread of cell phones, notebook computers and other portable electronic devices, there has been strong demand for the development of small, light nonaqueous electrolyte secondary batteries with high energy densities. Such secondary batteries include lithium-ion secondary batteries. Lithium metal and lithium alloys, metal oxides, and carbon and the like are used as negative electrode materials in lithium-ion secondary batteries. These materials allow the extraction and insertion of lithium.
Such lithium-ion secondary batteries are currently a focus of active research and development. Of these, lithium-ion secondary batteries using lithium-transition metal composite oxides and especially the relatively easy to synthesize lithium-cobalt composite oxide (LiCoO2) in the positive electrode material are promising and are being put into practical use as high-energy-density batteries because they provide high voltage in the 4V class. There has already been much development aimed at obtaining superior initial capacity characteristics and cycle characteristics with lithium-ion secondary batteries using this lithium-cobalt composite oxide (LiCoO2), and considerable success has been achieved.
However, lithium-cobalt composite oxide (LiCoO2) increases battery costs because it uses a rare and expensive cobalt compound as a raw material. Therefore, it would be preferable to use something other than lithium-cobalt composite oxide (LiCoO2) as a positive active material.
Moreover, recently there has been increased interest in the use of lithium-ion secondary batteries not only as small secondary batteries for portable electronics, but also as large secondary batteries for power storage, electric vehicles and the like. It is hoped that it will be possible to expand the range of applications by lowering the costs of active materials so that lithium-ion secondary batteries can be manufactured more inexpensively, and examples of materials that have been newly proposed as positive active materials for lithium ion secondary batteries include lithium-manganese composite oxide (LiMn2O4) using manganese, which is less expensive than cobalt, and lithium-nickel composite oxide (LiNiO2) using nickel.
In addition to using an inexpensive raw material, lithium-manganese composite oxide (LiMn2O4) is a promising substitute for lithium-cobalt composite oxide (LiCoO2) because it is thermally stable and especially because it is highly safe with respect to ignition and the like, but the drawback is that because its theoretical capacity is only about half that of lithium cobalt composite oxide (LiCoO2), it is unlikely to satisfy the ever-increasing demand for higher capacities in lithium-ion secondary batteries. Another problem is the extremely high rate of self-discharge at 45° C. and above, which lowers the charge-discharge life.
On the other hand, lithium-nickel composite oxide (LiNiO2) has roughly the same theoretical capacity as lithium-cobalt composite oxide, and exhibits a slightly lower battery voltage than lithium-cobalt composite oxide. Thus, more efforts have been made to the development of the former oxide, because decomposition due to oxidation of the electrolyte solution is unlikely to be a problem, and higher capacities can be expected. However, when a lithium-ion secondary battery is prepared using lithium-nickel composite oxide composed simply of nickel as the positive active material without substituting another element for nickel, the cycle characteristics are inferior to those obtained with lithium-cobalt composite oxide. Another problem is that battery performance is more likely to deteriorate when the battery is used or stored in a high-temperature environment. Furthermore, when the battery is left in a high-temperature environment in a fully charged state, the battery releases oxygen at lower temperatures than a battery using a cobalt composite oxide.
To solve such problems, there has been research into adding niobium, an element with a higher valence than nickel, to lithium-nickel composite oxides. For example, with the aim of improving the thermal stability of a lithium-ion secondary battery during internal short-circuit, Patent Document 1 proposes a positive active material for a nonaqueous secondary battery, consisting of particles having a composition comprising at least two or more kinds of compounds composed of lithium, nickel, cobalt, element M, niobium and oxygen as shown by LiaNi1-x-y-zCoxMyNbzOb (where M is one or more kinds of elements chosen from among the group of Mn, Fe, and Al, and 1.0≦a≦1.1, 0.1≦x≦0.3, 0≦y≦0.1, 0.01≦z≦0.05 and 2≦b≦2.2 are established), wherein the particles are roughly spherical and have a roughly spherical shell containing at least one or more kinds of compounds with a niobium concentration higher than that of the aforementioned composition on the inside or near the surface of the particle, and wherein the active material exhibits an electric discharge capacity of α [mAh/g] when the positive electrode potential is in the range of 2 V to 1.5 V during initial discharge, and simultaneously satisfies the conditions for α and β of 80≦α≦150 and 0.15≦β≦0.20 given β [deg] as the half-value width of face (003) of the layered crystal structure under X-ray diffraction.
Meanwhile, with the aim of improving thermal stability and increasing charge-discharge capacity, Patent Document 2 proposes a positive active material for a nonaqueous electrolyte secondary battery, represented by Li1+zNi1-x-yCoxNbyO2 (0.10≦x≦0.21, 0.01≦y≦0.08, −0.05≦z≦0.10), wherein the standard deviation of the intensity ratio INb/INi given INb as the peak intensity of the Nb L-rays and INi as the peak intensity of the Ni L-rays is within ½ the average value of the intensity ratio INb/INi as measured by the energy dispersion method.
Moreover, with the aim of providing large capacity and improving thermal stability during charging, Patent Document 3 proposes a positive active material represented by the compositional formula LixNiaMnbCocM1dM2eO2 (where M1 is at least one kind of element selected from the group consisting of Al, Ti and Mg, M2 is at least one kind of element selected from the group consisting of Mo, W and Nb, and 0.2≦x≦1.2, 0.6≦a≦0.8, 0.05≦b≦0.3, 0.05≦c≦0.3, 0.02≦d≦0.04, 0.02≦e≦0.06, and a+b+c+d+e=1.0 are established).
Meanwhile, with the aim of achieving both charge-discharge capacity and safety while controlling deterioration of the cycle characteristics, Patent Document 4 proposes a positive active material for a nonaqueous electrolyte secondary battery, having a structure comprising a lithium composite oxide represented by LixNi(1-y-z-a)CoyMnzMaO2 (where M is at least one kind of element selected from the group consisting of Fe, V, Cr, Ti, Mg, Al, Ca, Nb and Zr, and 1.0≦x≦1.10, 0.4≦y+z≦0.7, 0.2≦z≦0.5 and 0≦a≦0.02 are established), coated on the surface with a material A (where A is a compound comprising at least one kind of element selected from the group consisting of Ti, Sn, Mg, Zr, Al, Nb and Zn).
To achieve a large charge-discharge capacity with excellent thermal stability, Patent Document 5 proposes a positive active material for a nonaqueous electrolyte secondary battery, represented by Li1+zNi1-x-yCoxMyO2 and having two kinds of M impregnated in or attached to the material (where 0.10≦x≦0.21, 0.015≦y≦0.08, and −0.05≦z≦0.10, with M comprising two or more kinds of elements selected from Al, Mn, Nb and Mo whose affinity with oxygen is higher than that of nickel, and having an average valence higher than trivalence).
The proposals of Patent Documents 1 to 5 above are all aimed at achieving both thermal stability and charge-discharge capacity, but when the added amount of niobium is small the charge-discharge capacity is large but the thermal stability is inadequate, while when the added amount of niobium is large the thermal stability is good but it is impossible to ensure adequate charge-discharge capacity. Another problem is the difficulty of ensuring superior cycle characteristics.
While demands have been growing over years for higher capacities in the small secondary batteries of cell phone devices and the like, the advantage of high capacity provided by lithium-nickel composite oxides is lost when capacity is sacrificed to ensure safety. There is also a strong push to use lithium-ion secondary batteries as large secondary batteries, and especially as power sources for hybrid vehicles and electric vehicles and as stationary storage batteries for power storage. It is important that these batteries also have long battery lives and excellent cycle characteristics. For these applications, it is important not only to resolve the safety problems of lithium-nickel composite oxides but also to achieve high capacity and long battery life while maintaining a high degree of safety.